Living S cells (control)

Living R cells (control)

Heat-killed S cells (control)

Mixture of heat-killed S cells and living R cells

Mouse dies Mouse dies Mouse healthy Mouse healthy

Living S cells

RESULTS

EXPERIMENT

1

Bacterial cell

Phage head

Tail sheath

Tail fiber

DNA

100

nm

2

EXPERIMENT

Phage

DNA

Bacterial cell

Radioactive protein

Radioactive DNA

Batch 1: radioactive sulfur (35S)

Batch 2: radioactive phosphorus (32P)

Empty protein shell

Phage DNA

Centrifuge

Centrifuge

Pellet

Pellet (bacterial cells and contents)

Radioactivity (phage protein) in liquid

Radioactivity (phage DNA) in pellet

3

Sugar–phosphate backbone

5ʹ′ end

Nitrogenous bases

Thymine (T)

Adenine (A)

Cytosine (C)

Guanine (G)

DNA nucleotide

Sugar (deoxyribose) 3ʹ′ end

Phosphate

4

(a) Rosalind Franklin (b) Franklin’s X-ray diffraction photograph of DNA

5

(c) Space-filling model

Hydrogen bond 3ʹ′ end

5ʹ′ end

3.4 nm

0.34 nm

3ʹ′ end

5ʹ′ end

(b) Partial chemical structure (a) Key features of DNA structure

1 nm

6

Purine + purine: too wide

Pyrimidine + pyrimidine: too narrow

Purine + pyrimidine: width consistent with X-ray data

7

Cytosine (C)

Adenine (A) Thymine (T)

Guanine (G)

8

A T

G C

T A

T A

G C

(a) Parent molecule

A T

G C

T A

T A

G C

(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand

(b) Separation of strands

A T

G C

T A

T A

G C

A T

G C

T A

T A

G C

9

Parent cell First replication

Second replication

(a) Conservative model

(b) Semiconserva- tive model

(c) Dispersive model

10

EXPERIMENT

RESULTS

CONCLUSION

1 2

4 3

Conservative model

Semiconservative model

Dispersive model

Bacteria cultured in medium containing 15N

Bacteria transferred to medium containing 14N

DNA sample centrifuged after 20 min (after first application)

DNA sample centrifuged after 40 min (after second replication) More

dense

Less dense

Second replication First replication

11

Origin of replication Parental (template) strand

Daughter (new) strand

Replication fork

Replication bubble

Two daughter DNA molecules

(a) Origins of replication in E. coli

Origin of replication Double-stranded DNA molecule

Parental (template) strand Daughter (new) strand

Bubble Replication fork

Two daughter DNA molecules

(b) Origins of replication in eukaryotes

0.5 µm

0.25 µm

Double- stranded DNA molecule

12

Topoisomerase

Helicase

Primase Single-strand binding proteins

RNA primer

5ʹ′ 5ʹ′

5ʹ′ 3ʹ′

3ʹ′

3ʹ′

13

A

C

T

G

G

G

G C

C C

C

C

A

A

A

T

T

New strand 5ʹ′ end

Template strand 3ʹ′ end 5ʹ′ end 3ʹ′ end

3ʹ′ end

5ʹ′ end 5ʹ′ end

3ʹ′ end

Base

Sugar

Phosphate

Nucleoside triphosphate

Pyrophosphate

DNA polymerase

14

Leading strand

Overview

Origin of replication Lagging strand

Leading strand Lagging strand

Primer

Overall directions of replication

Origin of replication

RNA primer

“Sliding clamp”

DNA poll III Parental DNA

5ʹ′

3ʹ′

3ʹ′

3ʹ′

3ʹ′

5ʹ′

5ʹ′

5ʹ′

5ʹ′

5ʹ′

15

Overview

Origin of replication

Leading strand

Leading strand

Lagging strand

Lagging strand

Overall directions of replication

1 2

16

Template strand

5ʹ′

5ʹ′ 3ʹ′

3ʹ′

RNA primer 3ʹ′ 5ʹ′

5ʹ′

3ʹ′

1

1 3ʹ′

3ʹ′ 5ʹ′

5ʹ′

Okazaki fragment

1 2

3ʹ′

3ʹ′

5ʹ′

5ʹ′

1 2

3ʹ′

3ʹ′

5ʹ′

5ʹ′

1 2

5ʹ′

5ʹ′

3ʹ′

3ʹ′

Overall direction of replication

17

Overview Origin of replication

Leading strand

Leading strand

Lagging strand

Lagging strand Overall directions

of replication

Leading strand

Lagging strand

Helicase

Parental DNA

DNA pol III

Primer Primase

DNA ligase

DNA pol III

DNA pol I

Single-strand binding protein

5ʹ′

3ʹ′

5ʹ′

5ʹ′

5ʹ′

5ʹ′

3ʹ′

3ʹ′

3ʹ′

3ʹ′ 1 3 2

4

18

Nuclease

DNA polymerase

DNA ligase

19

Ends of parental DNA strands

Leading strand

Lagging strand

Lagging strand

Last fragment Previous fragment

Parental strand

RNA primer

Removal of primers and replacement with DNA where a 3ʹ′ end is available

Second round of replication

New leading strand

New lagging strand

Further rounds of replication

Shorter and shorter daughter molecules

5ʹ′

3ʹ′

3ʹ′

3ʹ′

3ʹ′

3ʹ′

5ʹ′

5ʹ′

5ʹ′

5ʹ′

20

DNA double helix (2 nm in diameter)

Nucleosome (10 nm in diameter)

Histones Histone tail H1

DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)

21

30-nm fiber

Chromatid (700 nm)

Loops Scaffold

300-nm fiber

Replicated chromosome (1,400 nm)

30-nm fiber Looped domains (300-nm fiber)

Metaphase chromosome

22